WAHM Wares is for stuff that you're selling from that thread, so you'd have to list your prices and quantities and update it when the stuff is sold, etc. WAHM Announcements is for a link to an off-DS sale or a "Hey I'm stocking today" etc. In addition, each user is limited to ONE thread in Announcements.

You can, but the items would then essentially be for sale in both places, leaving you at risk of selling the same thing twice. I'm sure there are members here who aren't registered shoppers over at HC (or don't even know it exists, lol) who would need to do transactions via PM or whatnot, so if an item sells at either place then you'd have to quickly update the quantity at the other place.